Preheat oven to 200 degrees F (95 degrees C). Grease a 14 x 18 inch cookie sheet.
Sprinkle one package of chocolate chips on prepared pan. Place in warm oven until chips melt, about 5 minutes. Remove from oven, and spread melted chocolate over bottom of pan; set aside.
In a large heavy saucepan over medium-high heat, combine butter and brown sugar. Stirring constantly, heat to 300 to 310 degrees F (149 to 154 degrees C), or until a small amount of syrup dropped into cold water forms hard, brittle threads. Immediately remove from heat. Stir in 3/4 cup slivered almonds and pour onto pan with melted chocolate; spread mixture evenly.
Sprinkle remaining package of chocolate chips over the almond layer. The heat from the almond layer will melt the chocolate chips; spread melted chocolate evenly. Sprinkle remaining 1/4 cup almonds over chocolate.
Cut into squares, or allow to harden in a solid sheet and break it apart like brittle. Cool completely before removing from pan.
Many cooks fail at making brittle by undercooking the sugar syrup or letting sugar crystals reform prematurely. Sugar confectionery relies on precise thermal thresholds and chemical interactions between sucrose, dairy fat, and roasted nuts to achieve the right snap. ### The Chemistry of Hard Crack Sugar When sucrose reaches temperatures between 300 and 310 degrees Fahrenheit, water evaporates almost entirely, and the remaining sugar molecules undergo caramelization while transitioning into the hard-crack stage. Butter introduces milk solids and water alongside milk fat. As the mixture boils, those milk solids brown through the Maillard reaction, developing deep toffee notes that balance the sweetness of the brown sugar. Using a heavy saucepan distributes heat evenly to prevent scorching the dairy solids before the sugar structure fully sets. ### Managing Fat and Emulsification High butter content creates an emulsion challenge during boiling. Constant stirring keeps the butterfat from separating out of the sugar syrup, ensuring a uniform candy mass. Pouring the hot, bubbling sugar directly over a pre-melted layer of milk chocolate creates a thermal bond, while the residual heat from the toffee layer softens the final top scattering of chocolate chips for smooth spreading. Cool the pan completely at room temperature before attempting to break the sheet into individual portions.